1. Field of Invention
The present invention relates to a process and transducers submerged in a fluid for emitting low frequency acoustic waves with lightened horns. The technical sector of the invention is that of the realisation of electro-acoustic submersible transducers.
The main application and the objective of the invention is the possibility, either to reduce the power consumption of a given submersible transducer comprising at least one horn and one motor pillar for emitting waves in a low frequency fluid, in fact lower than 500 hz, or to reduce the said frequency range of a given transducer for the same acoustic power, or both at the same time.
2. Background of the Invention
Such submersible electro-acoustic transducers are known and in particular the piezo-electric ones, comprising a hollow rigid cylindrical box, open at both axial ends, and within which two identical electro-acoustics motors are arranged coaxially with the latter, located on both sides of a central counter-mass, and whose opposite extremities are surrounded by a horn: these transducers are called double "Tonpilz". The said electro-acoustic motors can consist of two stacks of piezo-electric plates in line. The external faces of both horns are located in the plan containing the axial extremities of the box, so that they are in contact with the liquid, in which the box is immersed, and the external perimeter of these horns is close to the edge of the open axial ends of the said box.
Thus, these external faces emit acoustic waves into the liquid when the electro-acoustic motors are excited electronically; these transducers are notably used to emit low frequency acoustic waves in water, in a determined direction; for an application of this type of mono or double "Tonpilz" transducer to high power emissions, application FR. 2.663.182 from Mr Gilles GROSSO published on Dec. 13, 1991 can be mentioned, which describes the complementary devices to obtain an increase in power.
In order to avoid the propagation of the acoustic waves emitted by the horn rear faces, inside the box, especially when the latter is in fact full of liquid, and which are then retransmitted in the ambient medium in spite of the rigidity of the said box, various means are arranged in the cavity filled with ambient liquid to the rear of the horns of such non sealed boxes, such as elastic tubes closed at both ends and filled with gas, and such that the frequency of Helmholtz resonance of the cavity is near the fundamental frequency of the axial vibrations of the vibrating assembly; such a device is described in patent application FR. 2.665.998 of May 5, 1988 filed by Etat Fran.cedilla.ais, the General Delegate for Armament. The problem of the resistance is thus transferred to the pressure of the external box, to the resistance of the said elastic tubes, which, having smaller diameters, make it possible to have a less heavy assembly; other means can be developed in the same objective and be applied to the present invention, being known that these devices must provide a cavity, at the rear face of the horns, with sufficient dimensions.
Moreover, when, according to the objectives of the present invention, one wants to increase the electro-acoustic efficiency and therefore the power really emitted, which is increasingly requested presently especially in utilisations where the volume of energy storage is critical, while reducing the frequency under 500 hz, it is necessary to reduce the resonant frequencies of the motor and resonator; the resonant frequency being that of the transducer disunited from the acoustic load of the resonator. Usually, the reduction of the resonant frequency of the motor is obtained either by increasing the mass of the horns, or by reducing the section of ceramics of the motors (which in addition weakens them), thus reducing the maximum sound level emitted and which is contrary to the wanted objective. To avoid this power loss, it is therefore possible to choose to increase the mass of the horns and at the same time on the one hand the volume of the electro-acoustic motors, which results in an elongation of the latter and, on the other hand the rigidity and electro-mechanical coupling coefficient between the motors and horns; however the external volume of the transducer and its weight must then be increased, if not, a lower conversion of power is obtained; in any case, even if the power is increased at a low frequency, the acoustic efficiency remains average.
The problem is therefore the possibility of reducing the resonance frequency of a transducer motor for a given maximum power consumption and/or increasing its acoustic efficiency at low frequency in order to increase the acoustic power also without increasing the dimensions and weight.